This chapter is most relevant to Section F8(v) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the oxygen and carbon dioxide stores in the body". This has come up at least once in the First Part SAQS, disguised as a question on the effects of breathing 100% FiO2 (Question 1 from the first paper of 2011). Though it is not explicitly mentioned in the stem, the examiners tsked about how the candidates "simply lacked knowledge" when they failed to correctly guess that they were supposed to mention that oxygen stores are increased and discuss "the mechanism by which or extent to which stores are increased". On the basis of this, future candidates may have developed the impression that they also have to answer everything the question is not asking. Anyway, without any further whinge, the following brief summary will probably suffice:
Human bodily O2 stores consist of:
- Incorporated into body molecules:
- The human body is approximately 61-64% oxygen by weight
- 88.8% of body water weight is oxygen
- 11.3% of fat
- 22.7% of protein
- 41.4% of calcium hydroxyapatite
- Dissolved oxygen as molecules of O2 in blood and generally in body water
- Bound gas, complexed with other molecules:
- Bound to haemoglobin
- Bound to myoglobin
- Bound to other molecules
- Gas in cavities, particularly in the lungs:
- FRC is the most clinically important of these reservoirs
The distribution of oxygen in these stores is:
Form of storage O2 stores on room air (ml) O2 stores after 100% FiO2 preoxygenation As gas in the lungs (FRC) 270 1825 Bound to haemoglobin 820 910 Bound to myoglobin 200 200 Dissolved in tissue fluids 45 50
There really seems to be more work done on the oxygen stores of California sea lions than humans, if one is to believe the search results of Google Scholar. Cherniack & Longobardo (1970) would have been a definitive recommendation, but their excellent article is paywalled. Tanoubi et al (2009) and Nimmagadda et al (2017) give good accounta of denitrogenation/preoxygenation, and contain enough information to pass Question 1 from the first paper of 2011.
It will surprise nobody to learn that oxygen in the human body is so abundant that the human organism can be fairly described as a blob of gelatinised oxygen with some impurities. This makes logical sense because most people are 60% water by weight, and water is 88.8% oxygen by weight. As for the dry mass, oxygen is also 11.3% of fat, 22.7% of protein and 41.4% of calcium hydroxyapatite. So, add this up and you arrive at a pretty serious number. Snyder et al (1975), at the head of the official-sounding Task Group on Reference Man, reported that Reference Man was 61% oxygen by weight, probably on the basis of combustion data. In live men (half of whom had AIDS, in order to study the effects of weight loss on the oxygen content), Wang et al (1998) tried to measure this number directly by means of neutron activation whole-body detection, and arrived at very similar numbers (~63-64%). Obviously, this number, though interesting in a pub trivia sort of sense, is completely irrelevant clinically, because one simply cannot access most of this oxygen. It is hard to say that it performs no physiological role (see what happens to your physiology if you suddenly remove all the oxygen from those molecules), but it is certainly useless as a metabolic fuel.
Without digressing (even more) extensively, it will suffice to say that there are four reservoirs of oxygen in the body which one can potentially access as a source of metabolic fuel. These are:
The magnitude of these stores is unfortunately difficult to pin down, as all the textbooks give slightly different values and almost never give a reference as to where they have come from. Extensive inter-textbook plagiarism is exacerbated by the chapter author's confidence in the utter irrelevance of these data, thereby rendering their inaccuracy somehow safer and more excusable. Who cares what numbers you quote them, the grey-haired anaesthesia scholars may scoff over cognac; those young people will believe anything, and in any case I'm writing the exam questions. Anyway, the table below is an attempt to bring some accountability to this lawless wasteland.
|Form of O2 storage||O2 stores on room air (ml)||O2 stores after 100% FiO2 preoxygenation|
|As gas in the lungs (FRC)||450||270||500||370||430||3000||1825|
|Contained in blood||850||820||1200||880||1030||950||910|
|Bound to myoglobin||200||200||300||240||280||200||200|
|Dissolved in tissue fluids||50||45||56||60||100||50|
The Nunn values are borrowed from Nunn's textbook (8th ed., p. 194), which is the textbook you'd quote if you were writing a textbook. The Brandis values are reproduced from Kerry Brandis' excellent The Physiology Viva , which probably means the CICM examiners used them to study for their primaries twenty years ago. Brandis values are therefore exam-definitive, and they are also repeated in the chapter on the prevention of hypoxia during airway management, where body oxygen stores are related to the duration of time one can spend on re-acquiring the lost airway. The "C&L 1970" column comes from Cherniack and Longobardo (1970), a highly quoted article which seems to be referenced by the sort of physiology textbooks which feel the need to offer references. The Rahn values come from Henry Rahn's chapter ("Oxygen Stores of Man") for the 1964 book Oxygen in the Animal Organism, edited by Dickens and Neil - a compilation of papers presented by these people at an international symposium of physiologists in London. They are based on Rahn's earlier work (Farni & Rahn, 1955), which was an exercise of modeling these volumes mathematically. Cross et al (1968) tried to measure some of these volumes using radioactive 18O2 as tracer; the values used above are a combination of extrapolated dog findings, human measurements and theoretical modelling.